Sedimentation and shale tectonics of the northwestern Niger Delta front

Abstract

The modern-day slope and outer shelf of the Niger Delta are characterized by fault-bounded sedimentary depocentres and intervening shale structures, some of which are diapiric. Using two-dimensional seismic reflection data and stratigraphic information from five wells, the Neogene history of a 70 × 30 km portion of the northwestern Niger Delta shelf and slope has been examined. The shale structures were established by Late Miocene time, probably in response to lateral shale withdrawal from beneath the advancing deltaic load, combined with compressional uplift and folding of pro-delta strata. During Pliocene and Pleistocene time, these structures were buried by the prograding delta, and extensional growth faulting commenced. Stacked onlap relationships and stacked erosional channels indicate that the shale structures continued to influence sedimentation patterns throughout this period. Throughout the Pliocene, extension and subsidence were concentrated in a graben-like depocentre bounded by both regional and counter-regional growth faults. During the Early Pleistocene, subsidence on the regional growth fault ceased, and growth was transferred to the counter-regional growth fault set. The geometry and history of the depocentre is analogous to salt-related depocentres typical of the northern Gulf of Mexico and Atlantic margins of Brazil and Angola. To explain the history of the Niger Delta study area, a model is proposed in which the Miocene-Pleistocene sedimentary package has slid basinward, breaking up during extension, thereby creating seaward-stepping depocentres. Examples from analogue modelling studies are used to suggest possible origins for the large counter-regional growth faults.